This invention pertains to methods for treating atrial tachyarrhythmias. In particular, the invention relates to an apparatus and method for delivering shock therapy to terminate atrial fibrillation.
Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate, typically expressed in units of beats per minute (bpm). They can occur in either chamber of the heart (i.e., ventricles or atria) or both. Examples of tachyarrhythmias include sinus tachycardia, ventricular tachycardia, ventricular fibrillation (VF), atrial tachycardia, and atrial fibrillation (AF). Tachycardia is characterized by a rapid rate, either due to an ectopic excitatory focus or abnormal excitation by normal pacemaker tissue. Fibrillation occurs when the chamber depolarizes in a chaotic fashion with abnormal depolarization waveforms as reflected by an EKG.
Cardioversion (an electrical shock delivered to the heart synchronously with an intrinsic depolarization) and defibrillation (an electrical shock delivered without such synchronization) can be used to terminate most tachyarrhythmias by depolarizing excitable myocardium, which thereby prolongs refractoriness, interrupts reentrant circuits, and discharges excitatory foci. As used herein, the term defibrillation should be taken to mean an electrical shock delivered either synchronously or not in order to terminate a fibrillation. Implantable cardioverter/defibrillators (ICDs) provide this kind of therapy by delivering a shock pulse to the heart when fibrillation is detected by the device. An ICD is a computerized device containing a pulse generator that is usually implanted into the chest or abdominal wall. Electrodes connected by leads to the ICD are placed on the heart, or passed transvenously into the heart, to sense cardiac activity and to conduct the shock pulses from the pulse generator. ICDs can be designed to treat either atrial or ventricular tachyarrhythmias, or both, and may also incorporate cardiac pacing functionality.
The most dangerous tachyarrythmias are ventricular tachycardia and ventricular fibrillation, and ICDs have most commonly been applied in the treatment of those conditions. ICDs are also capable, however, of detecting atrial tachyarrhythmias, such as atrial fibrillation and atrial flutter, and delivering a shock pulse to the atria in order to terminate the arrhythmia. Although not immediately life-threatening, it is important to treat atrial fibrillation for several reasons. First, atrial fibrillation is associated with a loss of atrio-ventricular synchrony which can be hemodynamically compromising and cause such symptoms as dyspnea, fatigue, vertigo, and angina. Atrial fibrillation can also predispose to strokes resulting from emboli forming in the left atrium. Although drug therapy and/or in-hospital cardioversion are acceptable treatment modalities for atrial fibrillation, ICDs configured to treat atrial fibrillation offer a number of advantages to certain patients, including convenience and greater efficacy.
As aforesaid, an ICD terminates atrial fibrillation by delivering a shock pulse to electrodes disposed in or near the atria. The resulting depolarization also spreads to the ventricles, however, and there is a risk that such an atrial shock pulse can actually induce ventricular fibrillation, a condition much worse than atrial fibrillation. To lessen this risk, current ICDs delay delivering an atrial shock pulse until the intrinsic ventricular rhythm is below a specified maximum rate and then deliver the shock synchronously with a sensed ventricular depolarization (i.e., an R-wave). That is, an R-R interval, which is the time between a presently sensed R-wave and the preceding R-wave, is measured. If the R-R interval is above a specified minimum value, the interval is considered shockable and the atrial defibrillation shock pulse is delivered.
Currently available implantable cardiac rhythm management devices, including bradycardia and tachycardia pacemakers and cardiac defibrillators, have sense amplifier circuits for amplifying and filtering electrogram signals picked up by electrodes placed in or on the heart and which are coupled by suitable leads to the implantable cardiac rhythm management device. In some devices, the signals emanating from the sense amplifier are applied to one input of a comparator circuit whose other input is connected to a source of reference potential. Only when an electrogram signal from the sense amplifier exceeds the reference potential threshold will it be treated as a detected cardiac depolarization event such as an R-wave or a P-wave. The source reference potential may thus be referred to as a sensing threshold. Other devices implement the comparator function in software such that a digitized electrogram signal value is compared with a reference value in order to detect the depolarization event.
When a sensing threshold is set to a constant value, malsensing of cardiac depolarization events can occur due to a number of factors. First, cardiac depolarization events can have widely different peak amplitudes, depending on patient activity body position, drugs being used, etc. Lead movement and noise may further affect the detection of cardiac depolarization events. Noise sources may include environmental noise, such as 60 Hz power line noise, myopotentials from skeletal muscle, motion artifacts, baseline wander and T-waves. When noise levels in the electrocardiogram approach the sensing threshold, the likelihood of oversensing increases (i.e., false detection of depolarization events). If the sensing threshold is increased too high in an attempt to overcome the effects of noise, on the other hand, the likelihood of undersensing (i.e., failing to detect depolarization events) is increased. Methods have therefore been developed to automatically adjust the sensing thresholds of cardiac rhythm management devices in accordance with sensed activity. Such methods, however, present special problems with respect to R-wave detection in atrial defibrillators.
The present invention is a method and apparatus for delivering atrial defibrillation shocks synchronously with R-waves detected using a dynamic threshold with two sensitivities. In order to lessen the probability of inducing ventricular fibrillation, an atrial defibrillation shock is delivered in synchrony with an R-wave that occurs after a specified shockable time interval has elapsed as measured from the preceding R-wave. In accordance with the invention, ventricular electrograms are sensed through a ventricular sensing channel, and a ventricular depolarization event (R-wave) is detected when a sensed ventricular electrogram value exceeds a threshold that dynamically varies in accordance with measured peak amplitudes. In order to provide an R-wave detector with a desirable higher sensitivity within a short interval after an R-wave is detected and with a higher specificity after the shockable time interval has elapsed, two additional thresholds are employed to detect R-waves for purposes of delivering an atrial defibrillation shock. A low shock threshold with a low value relative to the dynamically varying threshold is employed for detection immediately after an R-wave is detected and thereafter until the shockable time interval elapses. After the shockable time interval has elapsed, a high shock threshold with a high value relative to the dynamically varying threshold is used to detect a subsequent R-wave.
In an exemplary embodiment, a dynamically varying threshold for R-wave detection is provided as a threshold function that is set to a specified percentage of the peak value of each detected R-wave and then decays to a base value (e.g., decreases exponentially or linearly). Upon detecting an episode of atrial fibrillation or other atrial tachyarrhythmia, an atrial defibrillation shock is delivered synchronously when the dynamically varying threshold detects an event, the interval from the last event detected by the low shock threshold to the event presently detected by the dynamically varying threshold or the low shock threshold is greater than the shockable time interval, and the high shock threshold also detects an event within a specified time interval from the event detected by the low shock threshold or the dynamically varying threshold. The low shock threshold and the high shock threshold may be specified as percentages of a specified minimum value for the dynamically varying threshold.